The extraction of heavy oil bitumen from an underground "reservoir" presents significant handling problems, by reason of the high viscosity of bitumen, and the presence of other liquids, gases and even solid particles in fluid admixture with the bitumen. Conventionally, pumping action is carried out using bladed impellers or vane-type pumps which pump the fluid to surface installations where subsequent separation of the fluid into its constituent parts takes place. High viscosity of bitumen, use of steam injection to lower the bitumen viscosity and abrasive materials result in many difficulties including solids impingement wear, and cavitation leading to pumping inefficiencies and incipient pump failures.
In a co-pending Canadian Patent Application No. 2,185,176, published on Mar. 11, 1998, the inventor previously disclosed a prior pump for handling viscous liquids, such as heavy oil bituminous fluid mixtures, and which overcomes some of the limitations of conventional vaned pumps. The inventor's prior pump utilizes a composite impeller "the prior impeller" having a stack of thin disks positioned concentrically over a cylindrical core. The disks are parallel and are spaced axially along the core. The core is formed with a plurality of upwardly spiraling vanes. The radial periphery of the core between vanes is open for fluid communication to the spaces between the disks. The core has a fluid inlet at one end of the vanes and fluid discharges at the periphery of the disks. The prior impeller is located concentrically within a cylindrical housing, forming an annular flow chamber therebetween. This stack of disks and the housing each have a cylindrical profile. In pumping operation, the core and disks are rotated. Boundary layer drag between pumped fluid and the rotating disks and centripetal force drives the fluid radially outwards to discharge at the disks' periphery and into the annular flow chamber. Fluid exiting the disks inducts fluid from the core's spiral vanes and from the previous impeller stage or pump intake.
A multiplicity of vortices are formed in the annular flow chamber. Like a centrifuge or cyclone, the fluid can separate into at least some of its separate component parts or phases, more dense fluid, such as contained solids, being driven outwardly. The vortices result in very unfavorable intake conditions should the fluid in the flow chamber be routed into the intake of a successive pumping stage. A stationary vane diffuser is applied between pumping stages. The prior impeller, while improving pumping capacity and performing primary separation, results in two phenomenon which are disadvantageous; high wear of the pump housing, particularly at the exit of the annular flow chamber, and high back-pressure at the impeller discharge which limits flow capacity.
At each downstream increment of the annular flow chamber, greater and greater accumulated flow is experienced. The accumulated flow results from each incremental increase in fluid exiting from each successive planer disk of the impeller. The linear increment in fluid discharge results in the development of back-pressure which affects the accumulating flow. Additionally, the combination of the incremental linear fluid discharge, the concentration of solids at the periphery of the flow chamber and turbulence results in high wear at the discharge of the flow chamber. The turbulence, the formation of discharge back-pressure and the housing wear result in reduced pump performance and increasing pumping inefficiencies.
This prior impeller is an improvement over other conventional impellers, and produces higher throughput and capability for handing mixtures including solids. However side effects, such as high housing wear, is an undesirable characteristic and, further, because multistage pumping can incorporate several hundred stages, the losses and back-pressure associated with each stage can be significant.